The invention relates to a test carrier for analyzing a sample fluid having several test layers arranged on the test carrier so that they are wetted in succession by the sample fluid. The sample is placed onto one of the layers, which is referred to as the sample feeding layer. The test layers form a fluid transport path for the test carrier.
Test carriers are used for analyzing sample fluids, in particular body fluids of man or animals. They are distinguished from the previously known analytical methods using liquid reagents above all by their ease of handling. The analyses can therefore also be conducted without specialized staff and near the patient.
There is embedded in the test layers a reagent system usually consisting of several reagents, in which the reagents are dissolved by the fluid and the reaction of sample and reagents leads to a detectable signal, in particular a color change in a detection layer. The detection signal can be visually or, preferably, instrumentally evaluated. Reflection photometry is typically employed to evaluate a color change. The invention is directed in particular towards test carriers which generate an optically measurable detection signal.
One problem with the performance of complicated analyses by test carriers is that the separation in time of several reaction steps is not a simple matter. The sample fluid usually flows along the fluid transport path largely uncontrolled.
The invention is directed towards a test carrier having a two-step process cycle, made possible by means of a separating layer. Such a test carrier is described in DE-C-31 30 749. A hydrophobic net, which is arranged in the fluid transport path between the reservoir layer of absorbent material and the detection layer, is used as the separating layer. In the previously known test carrier the reservoir layer, the hydrophobic net and the detection layer simply lie above one another on a band-shaped base layer of rigid plastic foil. They are bonded at one edge to the base layer. The reservoir layer is larger than the layers superimposed on it, so that it is only partly covered by the latter. The sample fluid is placed onto the part of the reservoir layer not covered by the detection layer and spreads out in the former beneath the detection layer. The hydrophobic net ensures that the detection layer is not immediately wetted by the fluid. In this state an initial reaction step can take place, the reagents being able to be contained in the reservoir layer or in further test layers which are situated immediately in front of the reservoir layer in the fluid transport path.
The fluid barrier formed by the hydrophobic net is only overcome if a pressure perpendicular to the surface of the layers is exerted, either manually or mechanically by a corresponding component of the evaluating instrument onto the layer assembly consisting of the reservoir layer, the hydrophobic net and the detection layer. The sample fluid then wets the detection layer and, where applicable, other test layers arranged between the hydrophobic net and the detection layer, and the proper detection reaction is initiated.
The use of a hydrophobic net separating layer permits a clean separation of two reaction steps on a test carrier. It is a disadvantage, however, that the color formation in the detection layer is in many cases not sufficiently uniform. This problem can be overcome by providing a thick reservoir layer and a correspondingly large sample amount. Additionally or alternatively the structure of the detection layer can be designed to ensure good lateral spreading of the fluid or an additional spreading layer may be located between the hydrophobic net and the detection layer. These measures lead however to an increased absorption of fluid by the test carrier. This is contrary to the goal of providing test carriers with as little fluid absorption as possible, because the required amount of sample fluid and the amount of reagent required in the test carrier are thereby reduced. Moreover, the detection layer should be as thin as possible in order to ensure intensive color formation. As a rule, however, a thin detection layer has poor lateral spreading and is therefore sensitive to non-uniform color formation.